1. Field of the Invention
This invention relates to the drilling or boring of wells in the earth. More specifically, a method is provided for characterizing the effectiveness of removal of cuttings to prevent sticking of the drill string in the drilling process, based on statistical analysis of combinations of drilling variables in prior wells.
2. Description of Related Art
At times during the drilling of wells in the earth, particularly deeper wells, the drill string cannot be retrieved from the wellbore, i.e., it becomes stuck in the wellbore. The stuck-drill string problem has been widely studied, and it has generally been concluded that the more common mechanisms which can cause the drill string to become stuck include: the hole becomes unstable and collapses around the drill string; a "key seat" of the width of the drill pipe diameter forms in the wellbore wall, such that larger diameter members of the drill string cannot be pulled through; segments of the drill string are held against the wellbore wall by excess hydrostatic pressure of the drilling fluid; and, solids build-up of cuttings in the wellbore around the drill string causes sticking. Those skilled in the art of drilling can often determine which mechanism caused the drill string to become stuck, based upon a review of observations before and after the sticking occurred.
To prevent build-up of solids in the wellbore, hole-cleaning of the wellbore must be achieved by the drilling fluid. In fact, one of the primary functions of the drilling fluid is to carry the "cuttings" from the bit where they are created to the surface of the earth. In vertical or near vertical wells, the drilling fluid is normally formulated to have viscosity high enough to decrease the settling velocity of cuttings to a value much less than the upward velocity of drilling fluid in the hole, so the cuttings will be efficiently carried from the wellbore. Gel strength of the fluid is also normally formulated to prevent rapid fall of cuttings in the wellbore when fluid circulation is interrupted.
In recent years, there has been a major upswing in the drilling of wells in directions other than vertical. "High-angle" wells are drilled for hydrocarbon production from platforms constructed offshore and from pads built in the arctic. "Horizontal wells," a sub-class of high-angle wells, are drilled at angles near 90 degrees to vertical for a variety of reasons related to hydrocarbon production; they may also be drilled for environmental remediation and other purposes. Some high-angle wells may terminate at a location displaced thousands of feet horizontally from the surface location of the well. There is a very large economic incentive at times to push this horizontal displacement to the maximum distance achievable so that additional hydrocarbons can be recovered from existing surface facilities.
It has been recognized for many years that removal of the cuttings from the wellbore during drilling of high-angle wells poses a special problem. The cuttings can settle by force of gravity along the bottom of the hole, since the settling velocity is no longer much less than the vertical velocity of the drilling fluid. As the cuttings settle in the drilling fluid, a "bed" of solids is formed along the bottom of the hole. This problem is especially severe in larger size holes, where fluid velocities are lower. Experience from drilling high-angle wells shows that pipe sticking and related drilling problems are especially frequent in the larger holes (171/2-inch and 121/4-inch holes) drilled at angles above about 40-50 degrees.
Removal of cuttings from vertical and intermediate angle wells can also present problems. In a vertical hole, it is believed that settling of cuttings when circulation or pumping of the drilling fluid is interrupted can cause the drill string to become stuck. At intermediate angles, cuttings may accumulate and then slide along the wellbore to enclose the drill string.
Build-up of cuttings in wellbores, or, stated another way, failure to achieve sufficient "hole-cleaning," can cause several types of problems. The most severe of these is sticking of the drill string, i.e., the drill pipe and equipment below it is stuck in the well. This condition can be very expensive to remedy. A single stuck pipe incident may cost over one million dollars. It is estimated that stuck pipe costs industry in the range from 100 to 500 million dollars per year. In attempts to avoid such problems from lack of hole-cleaning, drilling operators often include such maneuvers as "washing and reaming," wherein the drilling fluid is circulated and the drill string is rotated as the bit is introduced into the wellbore, and "backreaming," wherein the drilling fluid is circulated and the drill string is rotated as the bit is withdrawn from the wellbore. Other operations such as "wiper trips" or "pumping out of the hole" are performed to attempt to control the amount of cuttings accumulated in the wellbore. All these operations require time and can very significantly add to the cost of drilling a high-angle well.
Many studies of hole-cleaning in wells drilled at all angles from vertical to horizontal have been performed in the past. The article by S. S. Okrajni and J. J. Azar, SPE 14178, Society of Petroleum Engineers, Richardson, Tex., 1985, provides information on the subject. Several studies in university and industry laboratories in recent years have been directed to hole-cleaning in high-angle wells. Recent reports have been published, for example, by J. T. Ford et al, SPE 20421, Society of Petroleum Engineers, 1990, and by T. R. Sifferman and T. E. Becker, SPE 20422, Society of Petroleum Engineers, 1990.
It is common practice in the well drilling industry to record the values of the rheological properties and density of the drilling fluid, pumping rate of the drilling fluid, size and model of the equipment used in the drilling operation, and a variety of measurements of forces measured on the drill pipe, including, of course, the occurrence of sticking of the drill pipe in the hole. These values are normally recorded at least daily in a "morning report." In recent years large data bases containing these data for large numbers of wells have become available. Some such data bases are developed by oil companies or drilling companies and some by operators in certain geographic areas or geological provinces of the world. The existence of these data bases in machine-readable form and of computers makes possible the application of statistical techniques to large amounts of drilling data.
It has long been believed by many engineers in drilling organizations that hole-cleaning in wells will be improved, and hence the sticking of drill pipe due to lack of hole-cleaning will be lessened, by increasing the average velocity of flow of drilling fluid between the drill pipe and the wellbore wall. A data base of almost 400 wells drilled in the North Sea was used, along with well-known statistical techniques, to test this widely-accepted hypothesis. The results, shown in FIG. 1, demonstrate that the frequency of sticking of the drill string in the wells in this data base did not correlate with the annular velocity of drilling fluid in the wells. Obviously, other engineering variables must be sought which can predict sticking of drill strings.
U.S. Pat. No. 4,791,998 is directed to a method of avoiding stuck drilling equipment. The method is based on a statistical analysis of drilling variables in wells drilled in the same area, comparing wells in which sticking was experienced and not experienced, and modifying variables in a drilling well toward those conditions which the mathematical analysis indicates will not cause sticking. This patent addresses a variety of drill string sticking mechanisms. The variables which were believed to affect drill string sticking were combined in a purely statistical manner, using the implicit assumption that they combine linearly. The method disclosed in this patent does not include engineering information which accounts for the way each variable is believed to impact hole-cleaning, based on physical grounds.
There is a long-felt need for a method to determine if a well is being drilled under conditions where the drill pipe has a high risk of becoming stuck because of the accumulation of cuttings in the wellbore. The method should be applicable to wells drilled at any angle with respect to vertical. The method should couple engineering modeling and the large amount of drilling data available from prior wells. The set of engineering variables should describe the key mechanisms that govern the occurrence of stuck drill string and a method of combining engineering variables in a defined parameter should provide a statistical separation in the value of this parameter between the classes of mechanically stuck drill strings because of lack of hole-cleaning and non-stuck drill strings. A parameter which can easily be calculated should be provided and ranges in value of this parameter correlating to probabilities of sticking of the drill string should be applicable to wells drilled at any angle. The method should be applicable to planning the material and equipment which will be provided to drill a well and to adjust drilling variables while a well is being drilled. The method should be susceptible to continuous refinement as the data base expands or becomes more representative of the wells of interest, such that the risk of encountering drilling problems can be assessed with increasing confidence in a specific area.